Mining is an effective weapon, but it must be applied to suitable objectives, and its use restricted by certain rules which have been deduced from experience.

As regards the actual processes, the only important changes to be anticipated are the development of the use of rapid tools and of high explosives.

The most vulnerable points in trench-mine warfare are the following: Listening and observation posts in advance of the line; machine-gun and trench-mortar positions; junction-points of communicating-trenches with front line.

Mines were employed to attack, and countermines to defend, many besieged cities in the past, but the application of mining methods in trench warfare has at no time or place reached the scientific development which obtains in the present war, though our own Civil War furnishes several examples of its use in connection with the trench fighting of those days, viz.: Petersburg, Va., and other localities.

The situation of the western front is briefly as follows: Some thousands of mines have been blown by the Germans, British, and French, many of them with very large charges. The British and French were mostly engaged in defensive operations during 1915 and a portion of 1916, but since that date the application of mining methods by the Allies has been mostly on the offensive. Wherever opposing trenches are close together, mining warfare has almost invariably ensued. Mining fluctuates from time to time, and in 1915 and 1916 was most active on this front. It is always a possibility to be reckoned with, although the element of surprise, the most important factor in offensive mining operations, has now been largely discounted. On the other hand, the field of mining activity has been enlarged and is constantly changing. The destruction of enemy strong points is now an important problem for mining engineers. The use of mines in destroying roads and shelters also, in what might be described as the half-open warfare which has been so common this year, and in strategical retreats, is of increasing importance. The necessity of training engineer-pioneer units is important, as in the absence of special mining troops, this work may fall to their lot, particularly in the case of defensive operations in order to protect the infantry.

On account of the large number of men, quantity of material, expense, etc., the use of mining operations is very carefully considered beforehand, and careful plans devised before starting on any underground operations. Attacks are sometimes undertaken only to demolish a portion of the hostile trenches, in view of a local operation. At other times they form a part of a large offensive movement. Generally speaking, speed and silence are the first requisites. If these can be obtained with any degree of certainty, mining operations are nearly always successful. In the underground struggle in this war, the forces with superior personnel, material, and explosives at their disposal are bound to win, this is a fact proven by actual experience. The objectives will be usually such points as cannot be destroyed by artillery-fire, areas organized with deep dugouts and strong points which form valuable objectives, etc.

Almost everywhere on the western front the enemy are intrenched in deep, well-constructed trenches, and in nearly every area these trenches are further fortified by numerous deep dugouts and underground galleries, in most cases close-timbered with four-inch oak and other hard woods. The German mine systems are also almost invariably close-timbered.

In the same way as the general principles of tactics in open warfare agree with those of trench warfare, they are guided by the same logical reasoning. Many considerations must be studied before adopting any plan of offense.

With regard to depth below the surface, the miner must be guided by the elevation of his objective and by the nature of the strata through which he will have to work. In most operations the water-level is an important factor in considering depth.

It must be always borne in mind that mining is not an independent service, but is employed in connection with the other arms of the service.

Underground activity, either offensive or defensive, is first observed from those points in our lines nearest the enemy. All enemy trenches facing salients of our lines will be the object of particular attention and closest daily observation. This observation of the first-line and support-line trenches should disclose the presence of enemy underground works and their approximate location. For their definite location, the most careful listening underground must be done.

Aeroplane pictures are a source of much valuable information if studied very carefully. Aeroplane photographs covering several weeks, and if possible several months, are procured from the nearest squadron of the air service. The use of a strong magnifying glass will probably discover chalk mounds where fresh heaps are, by comparison with previous aeroplane photographs, shown to be growing in size; and other features or changes which will afford useful information on careful study. Particular attention is paid to new trenches in the enemy's lines. In fact, any change from week to week in these aeroplane photographs is carefully accounted for. If further pictures are desired, particularly just after craters have been blown by the enemy or ourselves, they are furnished by the air service. The enemy's lines are studied from day to day with field-glasses. Perhaps the accumulation of sand-bags different in color to the others will be noticed, or freshly placed earth, white and less dull in color than that of the parapets, which have been washed by the rain and blackened by explosives. As a matter of fact, in mining operations in Flanders, where blue clay was encountered, the bags containing the clay (different in color to surface clay) were so distributed along the front-line trenches or breastworks on both the enemy and British trenches that it was a matter of considerable difficulty to obtain much information from this source. It was, of course, common knowledge that both sides were engaged in mining and fairly accurate information as to position of enemy galleries was deduced, but the actual location of enemy mine-shafts was very difficult to determine, as quite a number of mine-shafts emanate from dugouts and are connected up with others.

Listening reports give a fair idea of where the enemy's galleries are, and plans are made up showing the suspected location of enemy galleries or mine system. These plans are developed in accordance with the evidence furnished. Careful observation may show that a number of men come up the communication-trenches regularly to certain spots and there disappear from view. One may notice carrying parties coming up with timber, by catching sight of the timber above the trench. Infantry listening patrols, who have been listening in "No Man's Land" at night may have heard the sound of machinery or ventilators at work, or some other sounds which go to prove the existence of enemy mining, or the location of mine-shafts, etc. It is important to listen in "No Man's Land" at night, in order to obtain definite information or confirm the reports from infantry.

Sufficient evidence being obtained as to probable location of enemy mine-shafts, or dugout entrances, etc., there is difficulty in having these places "strafed" by our own trench-mortars and artillery.

In the desire to obtain evidence of the enemy's activity underground, the proper camouflage of one's own spoil-heaps and entrances cannot be neglected.

It is essential to have a scheme of attack. Numerous attacks have failed on account of there being no clearly thought-out plan. A common mistake has been failure to anticipate possible counter-mining by the enemy, and to commence the attack without guarding against it.

Study of the objective and the ground will therefore furnish the information necessary to settle the essential elements of the scheme.

The attacks must be arranged so as to reach the objective in spite of anything the enemy may do. The vulnerable parts are the flanks; that is, both the flanks of galleries or branch galleries which are first constructed, and also the flanks of the combined scheme.

Usually it is of no advantage to begin an offensive by mining if the enemy has covered his position with countermines; if the distance to be covered is more than 150 yards; or if the water-level is very shallow.

There should be more galleries than the actual number needed to reach the objective.

They should be placed close enough to each other to insure enemy operations being detected from either parallel gallery.

If the enemy tries to bar the way by counter-mining, a breach must be made in his system, so that, in spite of him, the galleries may be driven to their objectives and charges placed as previously arranged.

The underground struggle which ensues must aim at clearing the ground by destroying the enemy's galleries, and at holding him to one part of the front whilst the attack passes elsewhere (to one side, above or below) by working quicker.

In order to do this successfully, it is necessary to get ahead of the enemy and strike before he can do so, to go for the flanks of the enemy's galleries, to strike as hard as possible, and to strike only when within good range.

To insure this it is required to have an accurate plan of the mines, a well-organized system of information, and decision and rapidity in execution. The value of a blow on the enemy's flank by placing the charges at the sides of branches or galleries will be enhanced.

In meeting the enemy underground, maximum-charge camouflets are largely employed. These will naturally vary with the depths at which they are placed.

To strike within good range requires great coolness. Every explosion causes a temporary delay in the attack, because it will damage part of the gallery leading to the charge. A mine should be fired only when it is considered that it is likely to do more damage to the enemy than to ourselves.

The preceding sketch illustrates a plan adopted by the French which may be used in suitable soil for the offensive. This procedure often gives good results in deceiving the enemy. He thinks he is protecting himself. The defensive gallery starts from the front line and the offensive from the support. Both galleries are in the same vertical plane, the second being more advanced in the direction of the enemy. In the top galleries very little attempt is made to deaden the noise, while work below is carried on silently. An enemy listener easily confuses one with the other, and the offensive gallery passes under him. Distances D and D¹ are the same. The miner M confuses the two sounds, and the offensive gallery passes under him.

(Note: In many of the districts in France, where a sandy clay forms the top-soil and a hard chalk the subsoil, this method could only be adopted with difficulty, as the conditions are reversed. The top gallery being in clay, it would be comparatively easy to do noiseless work there, but extremely difficult to carry on the lower chalk gallery without noise.)

Referring to sketch C, direct attack may be made in this manner, or this method might be used as a feint, while the enemy is driven around behind the gallery at another point.

The usual measures of precaution employed in ordinary warfare above ground must be taken below ground whenever the distance of the enemy and the nature of the soil do not exclude all possibility of a mining attack.

It is essential to have: An efficient lookout and listening service (observation of enemy's work above ground, and listening-posts above and below); Means of defense underground—countermines.

The most reliable information will be obtained by underground listening with portable listening-instruments, such as the geophone, etc. Much confirmatory evidence is obtained by listening patrols at night in "No Man's Land" and by a careful survey of enemy trenches.

Several different arrangements can be adopted. The fan-shaped arrangement as shown in Fig. B, or the arrangement of independent parallel galleries (Fig. C). The second arrangement is preferable. The interval between galleries varies with range of listening in different soils. In clay, the distances between galleries should not be greater than 60 feet. In chalk this may be safely doubled. Listening-galleries are usually put out in Y-form, and these galleries are of smaller size, often 3 feet by 2 feet in cross-section. If desired, holes may be bored from the ends of these listening-galleries and geophones placed in them. Where time, material, and personnel permit, the mine-shaft is put in at the support-line instead of from the front line. This can only be done, however, when the situation allows of it. The galleries in Flanders seldom reach a greater depth than 25 feet to 30 feet, but in the districts farther south, in the chalk country, mining operations are conducted at any depth from 80 to 150 feet. A comprehensive German mine system was found in the Somme district at 200 feet. Naturally, it is a distinct advantage to get one's own defensive system in first and then sit tight and listen for enemy work.

The best defensive is often a strong offensive. A concrete instance of this is furnished by the experience of a British mining company who were ordered from Flanders to trenches in the chalk district at the Vimy Ridge to meet what was termed "an urgent situation" underground. The Germans were mining from chalk galleries at depths varying from 60 to 100 feet, and inflicting heavy casualties on the infantry occupying the British trenches by blowing large mines under their advanced positions. As a result the infantry were obliged to abandon many of these forward positions. The hard chalk subsoil of this district was covered with a sandy clay top-soil varying in thickness on this sector from 1 to 30 feet in depth. Some defensive galleries in the chalk had been started by the French miners when occupying these trenches. The British at once undertook the construction of numerous galleries in the clay top-soil. Their rate of progress was much faster in clay than that of their opponents in chalk, in addition to which they could proceed without noise. The programme was daring but entirely successful. The enemy continued to handicap their efforts at the outset by blowing often, but within some six weeks the British had succeeded in completely establishing their mastery underground. A few camouflets were blown by them in crossing "No Man's Land," but the majority of their mines were fired directly below the enemy's first line, and in some cases beyond. So intense was the fighting that on some occasions the British blew as many as four mines a night on a 500-yard front.

The question of the blowing and occupation of craters is usually determined in consultation with the infantry staff. Some occasions occur, as, for instance, when the enemy is met below ground, when it is necessary to fire charges which will form craters; but, as a rule, no craters are blown in "No Man's Land" until the infantry have been warned and plans made for their occupation or otherwise.

Camouflets are employed to a large extent to destroy enemy galleries when they are met below ground. The infantry officers occupying the trenches are always informed of an impending "blow" from the enemy, and dispositions are made to withdraw any troops occupying dangerous positions, such as outlying bombers' posts or automatic-rifle detachments, etc.

All ranks in the mining companies are warned not to create an alarm among the infantry occupying the trenches when enemy "blows" are anticipated. Mining officers, at their discretion, advise the infantry of the state of affairs when there is very active fighting underground and there is danger of casualties among the infantry from enemy "blows." Only in the latter event is it wise to inform them. Under ordinary circumstances, no information should be given. Considerable success in warning infantry of impending mines by the enemy has been reached by the British and French miners. They often estimate the time of enemy mines being blown to within a few hours. On the other hand, the enemy often prepare charges and hold them for long periods before firing.

It is most important to ascertain, by underground listening, the nature of the work done. Determination of the nature of the sounds in listening is of equal importance to their location.

The practice of blowing craters on every occasion in "No Man's Land" is discouraged, generally on account of the fact that it will prove an obstacle in the assault when the next infantry attack takes place.

Many craters are blown for tactical reasons, such as the obtaining of good observation-posts and, in other cases, to obtain enfilade fire from the rim of the craters blown. Previous to blowing these craters, plans will have been made with the infantry to consolidate them as soon as blown. The exact diameter of the crater will have been calculated and the position located in "No Man's Land," and a trench constructed, probably the previous night, which will intersect the rim of the proposed crater. As soon as the mine is blown, the infantry will consolidate their positions on the rims of the craters. Many craters are used to good advantage in the disposal of spoil from near-by mine-shafts. Considerable difficulty is found in obtaining new places in which to dump the spoil, especially on active mining sectors.

Many devices are employed to deceive the enemy when firing. Dummy picks are often suspended in the face of the gallery and operated regularly to imitate picking, etc. Other sounds indicating progress of ordinary work may be simulated in the gallery being charged, or perhaps rather noisy work going on in an adjacent tunnel. A little ingenuity here is advisable. Quite a large number of mines are fired at "stand-to" at dawn or dusk, average time, 4 a.m. or 7 p.m., depending on the time of year. Naturally, it is not wise to make these times regular. The "stand-to" times are common because of the fact that the enemy infantry also occupies his fire steps at these times. Care must be taken to withdraw troops occupying dangerous positions.

In the event of blowing a series of mines under the enemy trenches, plans are devised with the infantry in an endeavor to induce the enemy to occupy his threatened trenches in force. This may be effected by a "false" infantry attack, and other means.

The mine is an irresistible means of launching an attack. In a mined sector, the best troops completely lose their bearings for several seconds after an explosion. These several seconds prevent the machine-guns from firing, and the assailant gains a foothold in the first line, and often in the second. The extent of the underground operations in launching an attack naturally depends upon the size of the offensive, whether it be a battalion, brigade, division, or army offensive. The date has been set for the general attack. Mining units, in consultation with the staff of the forces employed, will elaborate their plans for the mining operations. (The battle of Messines furnished an excellent illustration of the value and use of extensive mining in launching an attack. In these operations, the British miners fired a large number of mines from galleries under the enemy front line, and in some cases below their supports. The charges in these mines varied from 15 to 50 tons each, and were all fired at "zero" hour, the minute at which the infantry goes over the top. Nine hundred and fifty thousand pounds of ammonal were used. Some mines had been charged for fourteen months and others for a year. One charge of 95,000 pounds formed a crater 125 feet deep with 186 feet diameter. The largest crater formed was from a charge of 70,000 pounds, depth of crater 86 feet, diameter 260 feet. These mines were in chalk and clay. The result was a complete demoralization of the enemy, and the first objectives were obtained with very few casualties.) The Germans deserted their lines for half a mile to the rear. These mines were fired on a front of several kilometres. The scheme of the offensive having been planned, galleries are driven under the enemy trenches and all charges laid ready for firing.

Throughout the brigade or unit attacking, all watches carried by officers are synchronized so that at "zero" time all mines are fired simultaneously. Frequently mining units are detailed to go over with the infantry in raids, or follow them over in attacks, for special demolition work, in which event they will usually carry portable charges of high explosives, and destroy enemy mine-shafts, dugouts, etc.

The use of mining to destroy the numerous enemy reinforced concrete shelters is now coming to the fore. Artillery-fire, even of the heaviest character, often fails to destroy these shelters, and engineers are called on to destroy them by tunnelling and firing with high explosives. In most cases, it is usually essential for the success of the work that a sufficient depth of clay is found, in order to proceed without noise. On some occasions they have been destroyed from tunnels in chalk, but this is increasingly difficult with the enemy's improved methods of listening. Smaller galleries must usually be driven. It often happens that very little cover to these galleries can be obtained and the work is more hazardous, but the success of these operations is undoubted if careful measures are adopted.

In June, 1917, the writer's company destroyed a German reinforced concrete machine-gun post by tunnelling from the Hindenburg line for a distance of 200 feet, through an average of 7 feet of clay above the chalk. For most of the distance, only 2 feet of cover was possible, but the operation was completed in a few days, and with a charge of 500 pounds of ammonal, placed right up against the concrete, succeeded in entirely destroying this German defense, and largely assisted the infantry in capturing another 500 yards of the enemy Hindenburg trench.

In the instance referred to, the enemy had a listening instrument set up in the chalk, but had evidently failed to hear the work in the clay above.

Nearly all the German trenches are undermined by a series of subterranean galleries, thirty feet or more in depth. As they retreat, in addition to building blocks or breastworks in the trench on top, they will also blow the tunnels underground. It sometimes happens, however, that these underground galleries are not destroyed behind them, and the enemy have taken advantage of this situation by waiting until the trenches are occupied in force by the pursuing troops, and then blowing big mines under them. Careful investigation of the enemy trenches occupied after an advance is essential, though this obvious precaution is sometimes overlooked. In all mining work of this character, speed and silence are of first importance.